1. Field of the Invention
The present invention relates generally to exhaust systems for vehicles and, more particularly, to an exhaust system for a spark ignition engine of an automotive vehicle.
2. Description of the Related Art
It is known to provide spark ignition internal combustion engines for automotive vehicles. It is also known to provide exhaust systems for the spark ignition internal combustion engines. Commonly, the engine has exhaust manifolds to direct the exhaust gases to the exhaust system. Typically, the exhaust system includes exhaust intake pipes connected to the exhaust manifolds and a catalytic converter connected to the exhaust intake pipes. In the exhaust system, the exhaust gases flow through the exhaust intake pipes and into the catalytic converter. The catalytic converter has a catalyst substrate for converting potentially environmentally harmful exhaust gases passing therethrough into harmless by-products. To effectively and efficiently convert these exhaust gases to the harmless by-products, the temperature of the catalytic converter must be high enough to obtain a high rate of conversion. This is especially true when the engine is first started because the exhaust system is cold.
During cold start of the spark ignition internal combustion engine, fuel comes in contact with the cold combustion chamber and cylinder walls of the engine and does not combust or only partially combusts. In present exhaust systems, this heavy unburned fuel passes through the catalyst substrate unoxidized since the catalyst substrate has not been heated sufficiently (at least to 500.degree. F.) to ignite the hydrocarbon (HC). The amount of HC is significant, generally on the order of 1500 to 3000 parts per million (ppm) and counts heavily against the first bag count in federal test procedures. Although HC burners, electric heaters and by-pass traps have been used to warm up and activate the catalyst substrate prior to cold start or for trapping cold start HCs, they suffer from the disadvantages of adding a relatively high cost and complexity to the exhaust system.
Additionally during cold start, the exhaust gases that pass through the catalyst substrate have an air to fuel (A/F) ratio below stiochemetic (14.7). This results in a relatively high NO.sub.x output for the exhaust system. Although a lean NO.sub.x catalyst may be used on the catalyst substrate to reduce NO.sub.x, it suffers from the disadvantage that under ordinary conditions, the exhaust gas temperature is too high for this type of catalyst to yield adequate durability.